Such a method and such an apparatus are known from DE 10 2007 063 529 A1, for example.
The known method and the known apparatus use a multiplicity of different illumination patterns each having a spatial intensity profile having a defined period. In preferred exemplary embodiments, the illumination patterns are bright and dark stripes which form a sinusoidal intensity profile transversely with respect to the stripe direction. If such a pattern is displaced relative to the surface of a test specimen and if at least three images of the surface are recorded with the displaced stripe patterns, on the basis of the recorded images it is possible to determine different properties of the surface, in particular the local inclination of each surface point considered or a local scattering behavior of the surface point. However, the known method presupposes that the reflection properties of the surface examined allow a sufficiently distinct separation of the bright and dark stripes in the recorded images. The narrower the individual stripes of the illumination pattern, the more likely the stripes “blur” in the recorded images if the surface scatters diffusely. On the other hand, narrow stripes (corresponding to short periods of the intensity profile) enable a higher resolution and a higher measurement accuracy. For this reason, DE 10 2007 063 529 A1 proposes using a plurality of illumination patterns having intensity profiles having different periods in order to determine an optimum stripe width (period) in this way. In other words, the known method and the known apparatus use a multiplicity of illumination patterns having spatial intensity profiles having different periods in order to obtain a characteristic variable that is representative of the scattering characteristic of the surface examined.
The method described in DE 10 2007 063 529 A1 and the corresponding apparatus enable automated inspection of a test specimen having an at least partly reflective surface, wherein the prior knowledge about the properties of the test specimen that is required for the inspection can be reduced by comparison with even older methods. However, the known method and the known apparatus are still not optimal, since the stripe direction also influences the detection capability of the method and of the apparatus. Moreover, at least three (preferably at least four) images of the surfaces have to be recorded for each stripe width (period) and for each stripe direction, wherein the surface is displaced relative to the stripe patterns in each image. In the case of an unknown surface or a surface which can be characterized only with difficulty in advance, a high number of image recordings and shift steps are therefore required.
Nevertheless, the known method and the known apparatus do not yield complete characterization of an unknown surface, unless each surface point were recorded with a large number of illumination patterns with finely gradated stripe widths, stripe directions and shift positions.
WO 2005/031251 A1 discloses a similar method and a similar apparatus. The document also proposes the use of illumination patterns having different stripe periods in order, inter alia, to determine the reflectivity of the surface of a test specimen. With regard to the disadvantages, the same statements as those made in respect of the method and the apparatus from DE 10 2007 063 529 A1 are applicable.
There are a number of further known methods for inspecting a test specimen, wherein the test specimen is recorded together with a defined illumination pattern and the resulting images are evaluated. They include in particular stripe projection methods, wherein a stripe pattern is projected from a known position onto the surface of the test specimen in order to determine shape features of the test specimen on the basis of trigonometrical relationships. Furthermore, one method is known as Shape from Shading. This is a method wherein a test specimen is illuminated from different directions without changing its position relative to the camera, wherein shape features of the test specimen are determined on the basis of the different brightnesses depending on the light incidence direction. However, stripe projection and Shape from Shading are not suitable, or are suitable only to a limited extent, for test specimens having a highly lustrous surface. Furthermore, they do not yield a characterization of the test specimen surface, but rather only shape features.